A considerable amount of software has been written since digital computers were first used commercially. Much of this software, at least in its object code form, is not compatible with modern computing platforms, such as the Intel/Microsoft and Macintosh platforms which are increasingly used in commercial environments. This software, however, represents a significant investment for the companies that own it. As the computers for which this software was written become older and less reliable, these companies are faced with the task of recreating the functionality of these older programs in software that is compatible with modern-day platforms. While the cost of this effort may be justified for important software that is used in the day-to-day operations of a business, it may not be justified for less-important software.
One solution for handling this less-important software is to run it on an emulator for the older computer system. An emulator is a program configured to run on a modern-day platform that emulates the instruction set and architecture of another computer. In addition to the scenario for emulating legacy machines described above, there are various other reasons for emulating a computer system, including testing, or debugging, the program code associated with a computer system that is not physically available or which is difficult to program directly, for example, an embedded processor, or for evaluating different designs of a prospective computer system. To avoid confusion, in the materials that follow, the computer being emulated is referred to as the emulated computer or emulated system and the computer which is executing the emulation program is referred to as the host computer.
Computer emulation programs typically operate as interpreters. They fetch the object code of the computer to be emulated one instruction at a time and then perform functions equivalent to the fetched instruction on the host computer. The emulation program also includes data structures and a special-purpose operating system or enhancements to the operating system of the host computer that model the operating environment of the emulated computer.
Emulation of one computer architecture on another computer is inherently slow. Typically, an emulated program runs five or more times slower when compared to the same program running in the native mode of the host computer. Because, however, modern computers typically run programs much faster than legacy computers, there may be little or no difference in the execution of the emulated program on a high-speed modern computer and the original program on the emulated computer.
It is desirable, nonetheless, to provide an emulator which more efficiently models the emulated computer than a conventional interpretive emulator.